1. Field of the Invention
The present invention relates to a drop detection device that detects its own state when it is being dropped, and to a portable apparatus in which this drop detection device is installed.
2. Description of the Related Art
Portable telephones, digital cameras, and other such portable apparatuses are at a greater risk of being dropped from the user's hand or the place where they have been set during use than are non-portable apparatuses. In particular, with an apparatus equipped with a head and a disk, such as a magnetic recording medium, the impact of falling can cause the head and disk to collide, and this may damage them.
With drop detection devices (mechanisms) used up to now, drop detection was performed by utilizing the fact that the acceleration when a device is in free fall is approximately zero (however, the acceleration in a stationary state is 1 G (G=9.8 m/s2). In other words, a fall is detected when the threshold with respect to acceleration is assumed to be approximately zero, and this threshold is reached (or under it) and this state continues for at least a specific length of time. Then specific anti-impact measures are implemented (see Japanese Patent No. 3,441,668 registered on Jun. 20, 2003, for example).
With a configuration such as this, however, when a device falls while rotating, for example, rotational momentum may cause centrifugal force to act on the acceleration sensor built into the device, so that the acceleration ends up not being approximately zero.
Therefore, with a device having this configuration, if the device itself falls while rotating, there is the danger that the fall may not be properly detected. On the other hand, if the threshold with respect to acceleration is set relatively high so that a falling state will be more readily detected in order to solve this problem, then there is the possibility that this device will misinterpret vertical motion of the device produced by action on the part of the user as a falling state.
Also, in order to prevent such a misinterpretation, there is a drop detection mechanism having a constitution in which two thresholds for acceleration are set, and a time during which no drop detection is performed is provided when the acceleration satisfies specific conditions with respect to these thresholds (see Japanese Laid-Open Patent Application 2007-109355 published on Apr. 26, 2007, for example).
However, even with a constitution such as this, in the event that the user drops the device from eye level, for example, there is the possibility that the above-mentioned specific conditions will not be met. Therefore, even with a device constituted as above, there is the danger that the device will be mistakenly assumed to be in a falling state, so it cannot really be said that proper drop detection can be executed.